FIG. 5 shows a plastic shield plate for a bearing as disclosed in Japanese Laid-Open Utility Model 5-58955. Referring to FIG. 5, the shield plate E is disk-shaped and has a center opening E.sub.5 through which a shaft D and a bearing inner ring A extend. The plate E has a deep circular groove E.sub.2 formed in the surface of the plate extending outwardly from the bearing in the direction of the axis of the shaft. The outer side of groove E.sub.2 is bounded by a mounting part E.sub.1 which is received into a mounting groove B.sub.1 of the bearing outer ring B. The mounting part E.sub.1 has a flat surface E.sub.1a which abuts a horizontal surface B.sub.1a of mounting groove B.sub.1 when the shield plate is fitted into the mounting groove.
The shield plate is mounted on the bearing by placing the plate over the bearing (as viewed in FIG. 5) and pressing it downwardly. The plate flexes in the region E.sub.3 as the mounting part E.sub.1 is pressed against that portion of the outer ring bounding the upper portion of the mounting groove B.sub.1. If sufficient force is applied, the mounting part E.sub.1 snaps into the mounting groove.
Once the shield plate is in place, it is held by engagement of a flat surface E.sub.1b on the mounting part with a flat surface B.sub.1b bounding the mounting groove. To hold the shield firmly in place, the thin portion E.sub.3 of the plate, which flexes during installation, must be relatively stiff. However, if the portion E.sub.3 is too stiff then the outer ring is deformed as the shield plate is pressed into position.
High precision is required during the manufacturing of shield plate E and formation of the mounting groove B.sub.1. However, variations in dimensions may occur if, for example, there is a flaw in the cutting machine which forms the mounting groove B.sub.1 or the cutting machine is not accurately adjusted. The mounting part E.sub.1 is a stiff lump and lacks elasticity hence any dimensional variations in the groove B.sub.1 occurring during manufacture cannot be absorbed by the mounting part. If the mounting groove is too large, the shield plate may move loosely in the groove. On the other hand, if the mounting groove is too small, the outer ring may be deformed as the shield plate is pressed into position, and the main part E.sub.4 of the shield plate may be flexed so that it does not lie in a linear state normal to the axis of the shaft B but is instead inclined relative to the shaft.
If, to absorb any dimensional variations during manufacture, a soft synthetic resin material is used, the shield plate exerts less pressure against the surface of the mounting groove B.sub.1 so the shield plate may be detached by a slight external force.